Persons with familial adenomatous polyposis (FAP) have a nearly 100 percent lifetime risk of cancer, most often involving the upper or lower GI tract. Development of a cancer chemopreventive agent that could reduce this risk would offer a significant benefit. Nonsteroidal anti-inflammatory agents (NSAIDs) have shown efficacy in this disease by regressing prevalent colorectal adenomas. This trial seeks to determine the relative efficacy and tolerability of a promising agent combination (NSAID + difluoromethylornithine), versus an NSAID alone, versus placebo given over 6 months in 50 persons with FAP. The primary endpoint will evaluate the quantitative reduction in several parameters of colorectal adenomas following a six month period of administration. Other objectives include the evaluation of the treatments' efficacy against duodenal adenomas, and a panel of efficacy- and drug effect-biomarkers in duodenal and colorectal mucosa. Biomarker assessments are anticipated to improve mechanistic insights and iterative agent identification/development for this disease.